I know this forum may be a bit old ( Last post was in 2009), anyways, I have a question.

I'm developing a product base on the L-theanine, a product that anyone can use on a daily base.

My question was : What is the minimum quantity that someone should take to feel the effect of the L-Theanine ?

What is the exact feeling that this person will feel ?

Thank you in advance all of you for your answers !

I think the absolute minimum is 100 to 200mg. I have read it stated like that in otehr places when I was trying to figue out how much to take. I think it is common to take caps of 200mg twice a day. I do it only once a day. Supposedly it lasts about 8 hours and in my experience that is about right.
Really it depends on the person, but if I were making a product, like a drink, with some L-theanine, I would go with 200mg which is a very low and reasonable amount, but would probably be a lot more likely to give the desired effect to people. This would keep them coming back if they like your product's taste and want to have the implied benefits.

Also, I would look into getting Suntheanine, which I believe is often referred to as a superior version. Maybe better quality control? Not sure.